1. Fields of the Invention
The present invention relates generally to an apparatus for and a method of applying a radio frequency power supply to ignite a gas laser and operate it, and, more particularly, to such a supply which uses multiple, independent RF generators to provide different frequencies which enable optimal performance of each operation of the gas laser.
2. Discussion of Background And Prior Art
In order to obtain good performance of a CO.sub.2 gas laser with RF excitation, the RF power supply has to provide a high ignition voltage to the electrodes of the plasma tube to ignite the discharge in a laser gas mixture and, after ignition, it must provide high RF power to sustain the laser radiation output. These two independent conditions (maximum ignition voltage and maximum power) are difficult to achieve in actual practice because both ignition voltage and RF power depend on the output impedance of the power supply and the impedance of the tube. To further complicate the issue, the impedance of the tube is different for the two states of the tube (discharge ON and discharge OFF). Thus, once the RF power supply is tuned to provide maximum ignition voltage, the RF power would not be a maximum and vice versa. FIG. 1.
From an electrical point of view, the conventional tube for the laser with RF excitation is a resonant circuit with a capacitance and an inductance associated with the electrodes, and the ignition of the discharge is the equivalent of adding a resistance and a capacitance to the circuit. As a result, the impedance of the tube is lower with the discharge ON than it is with the discharge OFF, and the resonance frequency of the tube when the discharge is ON is about 1-3 MHz lower than the resonance frequency of the same tube when the discharge is OFF. These issues make the problem of the design of the RF power supply a difficult one. As described in greater detail below, numerous attempts have been made in the prior art to solve these complex problems.
a. Optimal Power Transfer At The Operating Frequency
To deliver the optimum power to the gas laser discharge the impedance of the oscillator output circuit should be matched to the impedance of the plasma discharge at its operating frequency. In U.S. Pat. No. 4,343,126 Chenausky taught that when the discharge is made a tuned circuit as by coupling a coil between the electrodes, and if the real (ohmic) impedance of the discharge is matched to the output impedance of the driving oscillator while the imaginary (reactive) impedance of the tube is canceled by careful choice of the coil, then, in that case, the optimal operating frequency is a frequency about 3% lower than the resonant frequency, and continuous operation at this lower non-resonant optimal frequency is characterized by a great increase in the power transferred to the discharge.
b. Dual Impedance Matching
However, as pointed out above, there is a significant difference in conditions of the laser plasma after plasma breakdown compared to before plasma breakdown. This difference manifests itself in the large decrease in the laser plasma tube impedance after plasma ignition. Importantly, an RF excited CO.sub.2 laser will not readily ignite if the rf excitation is initially applied at the frequency and power level which is optimal for full power continuous operation. Nourrcier U.S. Pat. No. 5,150,372 ("Nourrcier")
In an early attempt to solve the problem of requiring a much higher starting or ignition voltage than is required for normal operation or running, Sutter proposed in U.S. Pat. No. 4,455,658 using a power source having dual fixed element impedance matching circuits that have each been adjusted to provide a compromise between a first impedance match required for efficient, steady operation and a second impedance matching device coupling the electrodes to cancel the pre-ignition reactive impedance of the elongated chamber best for applying a high starting voltage to the electrodes. However, in actual practice, such compromise circuits cannot be optimized for either starting or efficient running.
As a proposed solution to this limitation, in U.S. Pat. No. 4,451,766 Angle ("Angle") attempted to provide an RF power supply that provides a high voltage for starting and also an impedance match between the power supply and the gaseous medium of the laser for high efficiency of energy transfer during steady state operation, by providing a variable impedance matching circuit controlled by a feedback voltage applied to a varactor diode. However, while this approach gives good performance at low RF power levels, it is unreliable at high RF power levels when the RF voltage on the veractor exceeds 300-500 volts.
c. Low Energy Ignition Pulses And High Energy Emission Pulses At The Same RF Frequency
A further proposed solution to control laser output power in a system which uses pulse-width modulation of the RF power applied to the laser plasma tube is discussed in Series 48 Lasers, Operation and Service Manual, (Synrad, Inc.), Release v. 2.0, Oct. 18, 1995, page 10 ("Synrad"). In order to shorten the delay between a user's ON command pulse and laser emission, Synrad delivers a short 1 .mu.s ignition pulse, having an energy level just below the laser emission threshold, at a 5 KHz pulse repetition rate which pre-ionizes the laser gas and a wider pulse thereafter which adds enough energy to the plasma to cause laser emission. This method allows the laser to respond predictably and almost instantaneously to the user's command signal. The problem with this approach is that the radio frequency of the short pulse is the same as the radio frequency of the long pulse so the ignition and operation of the laser are not optimized.
Thus, due to the inherent compromises necessary to provide two impedance levels matched to the amplifier, the high impedance prior to breakdown, and the low impedance after breakdown, an RF power supply tuned to a single frequency in a traditional implementation simply cannot satisfy optimum conditions for both ignition and pulsed or CW operation of a gas laser.
d. Single Fixed Frequency Generator With Manual Frequency Shifting
In an attempt to provide a multiple frequency power supply as a proposed solution to the problems experienced by Angle, Synrad, and others, Hesterman describes in U.S. Pat. No. 4,748,634 an RF power supply for gas lasers working on one frequency optimized for CW operation of a laser in which, in order to ignite the laser tube, the operating frequency of the single RF generator is momentarily shifted upwardly to the laser resonant frequency by changing the voltage of a variable capacitor in the bias circuit of the crystal oscillator to change the crystal frequency and thereby provide a large transient voltage for the laser ignition while using considerably less starting power.
While this approach is quite good for starting a laser and for CW operation and results in a smaller, lighter, and less expensive power supply, nevertheless, it suffers from low pulse to pulse reproducibility of the laser energy, especially in applications demanding a rapid and random change of pulse rate and pulse width of a laser (e.g., imaging, engraving and material cutting applications). The main reason for such lack of consistency in the pulse mode in this prior approach is the laser tube impedance dependency on the level and duration of the previous pulses.
e. Single Generator With Automatic Frequency Shifting Feedback Loop
Peter Laakman tried to solve Angle's problem in U.S. Pat. No. 4,837,772 by including the tuned circuit discharge in a circuit path feeding back via a quarter-wave impedance transformer 10% of the output power to the RF frequency power oscillator input which automatically self-adjusted its frequency depending upon the before or after ignition state of the discharge of the laser plasma tube. However, in this case, too, there is still a problem because it requires a very narrow range of the feedback parameters, a requirement that leads to significant difficulties in obtaining maximum efficiency and creates significant manufacturing difficulties.
f. Dual, Voltage Controlled Oscillators ("VCO's") Operating Open Loop On Two Different Frequencies.
An anonymous prior art circuit is described by Nourrcier (1:52-2:23) in which two VCO's connected in the open-loop configuration operate at different RF frequencies, and an RF switch selectively connects the oscillator outputs to the laser at the proper times. In this circuit one VCO initially has an output frequency higher than the igniting frequency and sweeps downwardly past the optimal continuous operating frequency while the other VCO generates the optimal continuous operating frequency. The RF switch connects the first VCO to the laser first for igniting, and then switches out the first VCO and switches in the second VCO after the laser has been ignited. Nourrcier points out the serious problems with this approach, namely, the significant output frequency variation resulting from the temperature drift which afflicts VCO's, and the difficulty in timely effecting switching so as not to let the laser go out as a result of having no RF output or the wrong RF output applied to it.
g. Frequency Sweeping Phase-Locked-Loop Synthesizer
Nourrcier attempted to solve the problems of the Synrad single frequency, pulse-width modulated circuit and of the anonymous two frequency, open loop circuit described above by using an RF power supply having a phase-locked-loop frequency synthesizer providing one frequency optimized for igniting the laser and another frequency optimized for CW operation of the laser. However, as in the anonymous circuit above, to achieve the ignition frequency Nourrcier's synthesizer swept the frequency of the drive signal downwardly from above to the higher resonant frequency optimized for laser ignition and then reverted to the still lower, full power, operating frequency.
While this approach may have solved the timing and temperature drift problems of the anonymous circuit and is quite good for starting a laser and for CW operation, it suffers from low pulse to pulse reproducibility of laser energy due to the laser tube impedance dependency on the level and duration of the previous pulses, especially in laser environments having rapidly changing, unpredictable pulse rates and pulse widths.
Thus, as amply demonstrated above, there is not available today, there is a long felt need for, and it is an object of the present invention to provide, a low cost apparatus and method which applies pulses at a first optimum radio frequency at an energy level below the emission threshold which efficiently ignite the laser gas and keep it ionized until pulses are applied at a second optimum radio frequency at an energy level above the emission threshold level which cause and sustain laser emissions while avoiding all of the problems experienced by the prior art solutions described above.